Electroetching is one of a number o f methods commonly used to remove metal from the surface of a metallic workpiece (i.e., a substrate) to provide the workpiece with an etched surface layer. In the electroetching method, the surface to be etched forms the anode of an electrolytic cell and a counter electrode (i.e., the cathode) is appropriately positioned to complete the cell. A suitable electrolyte is then directed onto the metal surface (i.e., the anode) while an electric current is applied to the system, leading to an accelerated etching of the metallic surface.
In the microelectronics industry, such electroetching is commonly used for through-mask patterning and for the removal of a continuous thin film of conducting metal (such as seed layers) from the surface of a semiconductor wafer. Different types of electroetching apparatus of this general type are known, and the literature describes a variety of electrolyte flow systems for use with such apparatus ranging from non-agitated to impinging electrolyte jets.
Completely submerged electroetching systems are often used for the electroetching of relatively large wafer layers, on the order of 200 mm in diameter. In practice, however, completely submerged electroetching systems tend to pose certain problems including high power requirements and edge effects that can lead to the loss of electrical connection with the metal film.
In order to alleviate such problems, U.S. Pat. No. 5,284,554 (Datta et al.) and U.S. Pat. No. 5,486,282 (Datta et al.) disclose an electroetching system which progressively treats only a small portion of the wafer to be etched. To this end, a multi-nozzle cathode assembly is provided which has a small width relative to the overall dimensions of the wafer (surface) to be etched. This cathode assembly is then used to deliver the electrolyte to limited portions of the wafer (i.e., the wafer surface facing the cathode assembly) as the nozzle assembly is scanned across the surface of the wafer. In this way, the surface area of the wafer which is at any given time exposed to the electrolyte is limited, and will be responsive to the location of the cathode assembly (i.e., the nozzle assembly) relative to the wafer being etched.
In operation, the electroetching system disclosed by the patents issued to Datta et al. is operated at a constant cell voltage, which in turn allows for adjustment of the etching current based on the exposed surface area which is encountered. Movement of the nozzle assembly is adjusted to match the rate at which the film is to be etched so that wafer etching can be completed in one pass.
Because conventional wafers have a generally circular shape, however, the scanning system of Datta et al. will encounter a surface area which varies as the cathode assembly is scanned across the wafer. A maximum surface will be exposed toward the middle of the wafer, while a minimal surface will be exposed at the ends (leading and trailing) of the wafer. This has been found to lead to nonuniformities which, in operation, can limit the overall etching rate for the wafer.
The deficiencies of the conventional electroetching systems show that a need still exists for an improved system. Therefore, the primary object of the present invention is to provide an improved electrochemical etching system for removing metal from the surface of a wafer. Another object of the present invention is to provide an improved electrochemical etching system for removing metal from the surface of a wafer at an increased rate. A further object of the present invention is to provide an improved electrochemical etching system for removing metal from the surface of a wafer which is well adapted to the shape of the wafer to be etched.